How to make G++ preprocessor output a newline in a macro?

Question

Is there a way in gcc/g++ 4.* to write a macro that expands into several lines?

The following code:

#define A X \\ Y

Expands into

Answer 1

Putting a side the fact that not being able to put newlines in macros create unreadable code, making it harder to debug preprocessor outputs. It is true that C and C++ might not care about newlines, but the C preprocessor does.

I would really like to make a macro ConditionalDefine(x,y) that outputs the following.

    #ifdef x
    #define y
    #endif

The following defines do something close:

    #define hash #
    #define nl
    #define _def_p8(A,B) A ifdef _P8_K60_BOARD_ A define B  A endif
    #define X_def_p8(A,B) _def_p8(A,B)
    #define def_p8(A) X_def_p8(nl hash,A)

expanding the following:

    def_p8(PTPD_DBGA 0)

results in:

    # ifdef _P8_K60_BOARD_ # define PTPD_DBGA 0    # endif

But without being able to put new lines in before the hashes it would not work as intended. It is also annoying the hoops you have to jump through just to get that close.